1. Field of the Invention
The present invention relates to novel cosolvent-free, aqueous, anionic polyurethane dispersions, to a process for their production and to their use as a lacquer, coating, adhesive or binder for any desired substrates, in particular as peelable lacquers.
2. Description of the Prior Art
Polyurethane plastics dispersed in water are known and have been described, e.g., in Anigewandte Makromoleculare Chemie, 98 (1981) 133-165). Polyurethane binders in aqueous lacquers have been disclosed in Wasserlacke, 1992, ISBN 3-8249-0100-5, page 46).
Because of their superior properties, such as flexibility, abrasion resistance, tear strength, as well as hardness and toughness, these polyurethane plastics may be used in a wide range of applications in the lacquers, coatings, adhesives and binders sectors.
Prior art aqueous polyurethane dispersions have made a major contribution to the stringent requirements to reduce emissions of volatile organic compounds (VOC's). However, prior art aqueous anionic polyurethane dispersions do generally still contain high-boiling, polar organic solvents such as dimethylformamide, N-methyl-pyrrolidone, etc. The addition of these high-boiling components simplifies the production process. Once the anionic polyurethane plastics have been converted into the aqueous phase, the high-boiling components remain in the dispersions. This is desirable when these dispersions are used as lacquers, coatings or adhesives, because the organic, polar compounds act as plasticizers, levelling agents and/or coalescing agents and widen the range of applications for the dispersions.
Disadvantageously, these high-boiling components evaporate from the layers over the period of use as a lacquer, coating or adhesive and the organic, polar high-boiling components reduce the water resistance of the lacquer, coating and adhesive layers.
When the dispersions containing cosolvents are used as a peelable coating for the temporary protection of coated surfaces, organic, polar, high-boiling solvents are undesirable because they damage the coated surfaces. These high-boiling components are also undesirable for when the coatings are applied to plastics.
Peelable coatings for the temporary protection of, for example, motor vehicles, are known. Japanese application JP 05 171 068 describes peelable coatings consisting of polybutyral resins, plasticizers, lecithin, organic solvents, non-sublimable hindered phenols, anti-settling agents and UV absorbers. U.S. Pat. No. 5,330,788 describes an aqueous coating composition which contains a film-forming acrylate polymer and surface-active compounds.
The coating composition contains alkanolamines and "surfactants," i.e., surface-active substances. An anionic phosphate ester is added as the "surfactant." An aqueous concentrate of alkanolamines, surface-active compounds and chelating agents is used as the stripping agent. No information is given with regard to the fate of the stripped coating waste. An automotive coating method to protect the vehicle against stone chipping is also described. The coating used in this method is a polyurethane coating described in no further detail dissolved in isopropanol, toluene, ethyl acetate mixtures. Once damaged, the lacquer may be stripped and the automotive component then recoated with the coating solution.
The previously described processes for the temporary protection of coated or uncoated articles are associated with serious disadvantages. On the one hand, organic solvents are used for the lacquer resins, which can have a detrimental effect on the coated surfaces. On the other hand, stripping agents are used, some of which contain strongly basic compounds. No information is given with regard to the fate of the stripped lacquers.
Recyclable peelable coatings are previously unknown. Reference may here be made, for example, to the automotive industry's demands for aqueous peelable coatings to protect the exterior of motor vehicles. Wax-based exterior protection agents are no longer acceptable in various countries on environmental protection grounds.
The performance currently required from peelable coatings is that the coating should be water-based and cosolvent-free. The coating solution or dispersion should be capable of being applied by dipping, knife coating, pouring, brushing and spraying (airless and electrostatic).
Subsequent drying at 20 to 80.degree. C. should yield a homogeneous film. The film should be water resistant, transparent, tear-resistant, UV-resistant, temperature-resistant, resistant to precipitation (of an organic or inorganic nature), and should adhere to the substrates and be readily removable by peeling. The coating should be reusable by means of a suitable process.
Previously known methods, processes and coatings do not fulfill all of these requirements. It must thus be considered surprising that the aqueous polyurethane-polyurea dispersions according to the invention fulfill all of these requirements, especially since it is known that polyurethane coatings are particularly suitable for the production of coatings and multi-layer coatings having good adhesion properties as disclosed, for example, in EP-A 634,431. The thermoplastic poly-urethanes having lateral carboxyl groups described in U.S. Pat. No. 4,139,674 adhere so strongly to glass that they are ideally suited to the production of glass laminates.
It is thus particularly surprising and unexpected that the polyurethane-ureas according to the invention do not exhibit this characteristic, provided that certain drying conditions are maintained.
It has not been disclosed that the aqueous polyurethane dispersions described in Angewandte Makrornoleculare Chemie, 98 (1981) 133-165 are peelable coatings. EP-A 669,352, EP-A 407,798, EP-A 427,979 and EP-A 269,972 are among the closest prior art. EP-A 269,972 describes polyurethanes, which are soluble or dispersible in water and contain 0.5 to 30 wt. % of ethylene oxide units present in polyether chains arranged terminally or laterally and 0.1 to 120 mnilliequivalents of carboxylate groups per 100 g of solids. The products of the process are suitable for coating textiles, leather, paper, etc. They are not suitable for peelable coatings because they adhere too strongly to the substrates and swell when exposed to water. This is a disadvantage common to all polyurethane dispersions containing hydrophilic polyether chains in the solids. U.S. Pat. No. 3,412,054 describes polyurethane dispersions having limited dilutability in water and which require the presence of a cosolvent.
The products described in EP-A 427,979 have very good coating properties, such as adhesion and weathering resistance, but due to their content of hydrophilic monofunctional polyether chains are unsuitable for use as a peelable coating.
EP-A 407,798 describes a process for the production of aqueous dispersions of polyurethanes and their use as coating compositions for any desired substrates. Trialkylainmonium compounds are used in this process in order to prevent the isocyanate groups and ammonia from reacting. The process is elaborate and, in addition to unsubstituted ammonium ions, the products also contain trialkylammonium counterions.
DE-A 4,343,895 describes a process for the production of polyurethane dispersions by reacting prepolymers containing unneutralized carboxyl groups and/or terminal NCO groups with ammonia and subsequently dispersing them in water. Ammonia is used in chemically bound form as ammonium carbonate. It is disclosed in the examples that N-methylpyrrolidone is used as a cosolvent. The products are used for the production of coatings, which are preferably crosslinked with melamine resins, for solid surfaces. These products are unusable as a peelable coating, due to the presence of NMP as the cosolvent and because they do not form peelable films without crosslinking agents.
EP-A 669,352 describes "soft-feel" coatings having good mechanical film properties and adhesion. It is disclosed in the examples that the products of the process have a content of organic solvents, in particular NMP, of approx. 2 to 3%. Aqueous polyurethane resins containing solvent are unsuitable as peelable coatings because they attack the coating layers they are intended to protect.
Cosolvent-free, aqueous dispersions of polyurethane-ureas having lateral carboxyl groups, which are at least partially neutralized by ammonia, containing no other hydrophilic groups and coating sheets peeled therefrom or can be recycled to aqueous dispersions have not previously been described.
An object of the present invention is to provide anionic polyurethane plastics, which may be used to provide coatings and adhesives which fulfill requirements with regard to mechanical properties, weathering resistance, light fastness, transparency, temperature resistance, tear strength, elongation, water resistance and resistance to precipitation (of an organic or inorganic nature) and recyclability.
It has now surprisingly been found that the type of salt groups bound to the polymer and the average particle size of the dispersed polymer particles are significant for the formation of defect-free layers. The process according to the invention allows the person skilled in the art, by means of these parameters, to reproducibly obtain stable anionic, cosolvent-free polyurethane dispersions and to produce defect-free coatings therefrom, especially on industrial coating lines.